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Reducing power of three-coordinate cobalt(I).
Michael J Ingleson1, Maren Pink, Kenneth G Caulton
1Department of Chemistry, Indiana University, Bloomington, 47405-7102, USA.
Journal of the American Chemical Society
|March 30, 2006
Summary
Cobalt complexes with PNP ligands readily bind carbon monoxide, indicating an easily oxidized cobalt(I) center. However, full oxidation to cobalt(III) is challenging, with intermediate species forming instead.
Area of Science:
- Organometallic Chemistry
- Coordination Chemistry
- Inorganic Synthesis
Background:
- The reactivity of low-valent transition metal complexes is crucial for catalysis and materials science.
- Cobalt complexes, particularly with phosphine ligands, are of interest due to their diverse oxidation states and bonding capabilities.
- Understanding the electronic properties and oxidation pathways of cobalt complexes informs the design of new functional molecules.
Purpose of the Study:
- To investigate the coordination chemistry and oxidation behavior of a cobalt complex supported by a bulky PNP ligand.
- To characterize the products formed upon reaction with carbon monoxide and oxidizing agents.
- To explore the limits of cobalt oxidation within this specific ligand environment.
Main Methods:
- Synthesis and characterization of the (PNP)Co complex.
- Infrared spectroscopy to study carbon monoxide coordination.
- Reactions with iodine (I2) and a diazo compound (N2CH(SiMe3)) to probe oxidation.
- X-ray crystallography to determine the structure of reaction products.
Main Results:
- Carbon monoxide addition to (PNP)Co yields (PNP)Co(CO), with a nuco value of 1885 cm-1, indicating significant back-donation and an easily oxidized Co(I) center.
- Oxidation of (PNP)Co with I2 does not lead to a stable Co(III) species; instead, (PNP)CoI and a zwitterionic Co(III) complex, [ItBu2PCH2SiMe2NSiMe2CH2PtBu2]CoI2, are formed.
- Reaction with N2CH(SiMe3) results in a 1:1 adduct whose crystal structure suggests partial oxidation of Co(I), short of a full Co(III) state.
Conclusions:
- The bulky PNP ligand stabilizes a readily oxidizable Co(I) state, facilitating CO binding.
- The cobalt center's accessibility to higher oxidation states, like Co(III), is sterically and electronically limited by the PNP ligand and reaction conditions.
- The observed reactivity highlights the nuanced control over redox properties achievable with tailored ligand design in organometallic chemistry.
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